1. Field of the Invention
The present invention relates to discrimination of features at depth within the earth from features close to the surface during electromagnetic surveying by injection of electrical current into the earth with selectable different sets of electrodes.
2. Description of the Related Art
Electromagnetic (EM) geophysical soundings probe electrical resistivity, or equivalently, conductivity, in the ground as a function of depth. Typical targets of interest include ore bodies, hydrocarbons, water, and environmental pollutants. Since the resistivities of such targets and the surrounding medium may be quite dissimilar, they may be discriminated by means of measurement of their subsurface resistivity when subjected to an electromagnetic field. Using this methodology, the depth, thickness, and lateral extent of materials of interest may be determined.
The source of the EM field used in a geophysical sounding may originate in the natural environment, or be manmade. If man-made, the source may produce a primarily a magnetic field or electrical field that varies in time. Such a primary field also produces a secondary field in the conducting earth. For example, an electrical field produces electrical currents in the earth that have an associated magnetic field, and a time-varying magnetic field induces electrical currents that result in an electrical field.
The electrical properties of the earth and rate of change of a field determine the relative magnitudes of the primary and secondary fields. The resultant primary and secondary fields represent a combined electromagnetic interaction with the earth even for a source arranged to produce solely an electrical or magnetic field.
While the majority of EM geophysical soundings are performed with sensors and EM sources on the surface of the earth, a borehole can provide physical access to the subsurface. Measurement of the electrical or magnetic field within a borehole can be related to the electrical or magnetic field in the earth around the borehole, or the fields that would exist in the earth in the absence of the borehole. Similarly, connecting an electrical field or magnetic field source to the earth via a borehole provides away to produce fields within the earth at desired depths without the attenuation and uncertainties that may result if the source fields originated from a source at the surface of the Earth.
The borehole may be an open hole in the host rock or may include a casing. If a casing is present it is usually made from a metal alloy, in which case it has a low value of electrical resistivity, or it may be made from an insulator, such as fiberglass. In some situations the casing may be segmented so that is partly a good electrical conductor in one or more regions and partly an insulator in one or more regions. Further the borehole may have tubing inserted into it, such as production tubing that affects the path of electric current.
A particularly beneficial configuration of borehole EM source is an electrode situated at the approximate depth of the formation or target of interest and a counter electrode situated at the surface adjacent to the well. Electric currents are caused to flow between the two electrodes with a suitable transmitter. These currents flow outwards radially from the well, probing a lateral region of order the depth of the downhole electrode. In another example, the counter electrode adjacent the well is replaced by a suite of six or more counter electrodes are arranged in an approximate ring or circular pattern centered on the borehole and of radius or order the borehole depth. In this case, significant electric currents in the ground are caused to flow from a source electrode at depth outwardly a radial distance from the borehole generally corresponding to the radius of the counter electrode ring. In yet another configuration a ring of counter electrodes is used, as described above and the electrically conducting casing of a borehole is used to inject the current at depth. In this case contact can be made at the top of the borehole.
The distribution of electric current flow produced by an EM source is determined by the three dimensional resistivity distribution within the earth. The electric current measured at the surface, or at depth with a borehole can be used to infer the 3-D resistivity variation over the region where significant current is flowing. The current is typically measured by a suitably calibrated array of electric or magnetic field sensors. The resulting 3-D resistivity variation can be used to project the distribution of ores, hydrocarbons or water within the volume of interest under investigation during the survey.
A common problem in applying the method of subsurface EM imaging has been to discriminate in the EM survey those data or measurements of interest, which are spatial changes in resistivity at the depth of the formation, from the effects of measurements occurring near the surface location of the sensor array. For an example array of electric field sensors deployed at the surface and a hydrocarbon reservoir at depth greater than 1 km, the challenge has been to reliably and accurately separate resistivity anomalies due to near surface inhomogeneities from spatial variations of interest for evaluating reservoir resistivity. This has been a particular challenge because the sensors are located very close to the earth surface and correspondingly far from the reservoir. The problem has been even more difficult in the case of time lapse monitoring, because the resistivity at the surface can be significantly affected by weather events such as rainfall and temperature variations occurring during the passage of time.